Yet as the virus spreads, the rallies and face-to-face interactions that give life to their campaigns will seem riskier. After scrapping tonight’s rally, the Sanders campaign said future events would be evaluated “on a case by case basis.” If they resume, questions remain: Should Biden and Sanders discourage older Americans from attending their events, for example? Should they themselves cut off all physical contact with their supporters?

“The people of the United States,” Sanders said in Detroit yesterday, “want to know that we have an administration in this country whose decisions and comments are based on science, not based on tweets that have no scientific basis, not based on politics.”

For Sanders and his main rival, living up to that ideal could soon lead to some very uncomfortable decisions. Should the Democratic candidates merely follow the guidance and model of a president whose decisions they’ve denounced? Or should they lead by example?

“We ought to worry about everybody, but particularly about seniors,” Representative Donna Shalala of Florida, the former two-term secretary of health and human services under President Bill Clinton, told me when I asked her whether she was worried about Biden and Sanders. “I would like to see them do the same thing that we’re telling everybody else about large gatherings, about public-health measures,” she said. “They should not do anything that everybody else is not doing.”